1. Technical Field
The present invention relates to an ink jet recording method, an ultraviolet curable ink and an ink jet recording apparatus.
2. Related Art
In the related art, as a recording method for forming an image based on an image data signal on a recording medium such as paper, various methods have been used. Among them, since an ink jet method is used with an apparatus with a low cost which discharges ink only to a necessary image unit and performs direct image formation on a recording medium, it is possible efficiently to use the ink and to keep the running cost is low. Furthermore, since noises are small in the ink jet method, the ink jet method is excellent as a recording method.
In recent years, in order to form an image having excellent water resistance, solvent resistance, rub fastness and the like on a surface of a recording medium, an ink jet recording method using ultraviolet curable ink which is cured when being irradiated with ultraviolet light has been used.
For example, JP-A-2010-131975 discloses a line recording type image forming method of discharging a photoacid generator solution PI-1 (CPI-100P manufactured by San-Apro Ltd., a photoacid generator solution of sulfonium salt which is dissolved in propylene carbonate) onto a recording material in advance, subsequently irradiating the photoacid generator solution which is landed onto the recording material with ultraviolet light from a metal halide lamp (VZero270 manufactured by Integration Technology, the maximum illumination of 400 mW/cm2), discharging cationic polymerization-based active light curable ink (in order of black, cyan, magenta, yellow and white) formed of a photoacid generator (photo radical generator is not used), a cationic polymerization compound, a pigment dispersion, a surfactant and a sensitizer, each of which has predetermined types and amounts sequentially, to form an image, and then, performing irradiation with active light from an LED (manufactured by Nichia Corporation, 365 nm-multi chip array, a water cooling type, the maximum illumination of 1,500 mW/cm2) to fix an image (Paragraphs [0028], [0088], [0094] to [0097], [0099] to [0101] (Table 1), and [0106], Sample No. 1 in Table 5 in [0107], FIG. 1, and Paragraph [0112] in JP-A-2010-131975).
In addition, for example, International Publication No. WO 2011/039081 discloses a recording method of discharging ultraviolet curable ink jet ink consisting of 15% by weight of a dispersion liquid including C.I. Pigment Blue 15:4 which is a pigment, 62.55% by weight of propoxylated neopentyl glycol diacrylate (SR9003), 13% by weight of 2-(2-vinyloxy ethoxy) ethyl acrylate (VEEA), 0.83% by weight of a polymerization inhibitor, 6% by weight of a photopolymerization initiator, 2.5% by weight of ethyl-4-(dimethylamino)benzoate, and 0.1% by weight of polyether-modified polydimethylsiloxane which is a wetting agent from a print head in which density of nozzles and an outer diameter thereof are set as the predetermined values, and performing irradiation with ultraviolet light (Example 1 in International Publication No. WO 2011/039081).
However, when manufacturing a cured film, that is, a coating film of cured ink, from ultraviolet curable ink jet ink using the image forming method which is disclosed in JP-A-2010-131975, the following problems occur.
When a coating film of ink is irradiated with ultraviolet light, a pigment strongly tends to absorb a part of ultraviolet light, and accordingly, although irradiation is performed with the ultraviolet light, an energy necessary for the complete curing of the coating film which is discharged onto a recording medium, is insufficient, and thus the vicinity of the surface of the coating film is initially cured, and thus, the inner portion of the coating film is incompletely cured or more time is necessary for the curing, in some cases. In addition, when the uncured ink existing in the inner portion of the coating film is cured, wrinkles are generated on the vicinity of the surface of the coating film which is initially cured, or by irregular flow of the ink before the ink in the inner portion of the coating film is cured, wrinkles (hereinafter, referred to as “cured wrinkles”) are generated on the surface of the coating film after the curing. Due to the cured wrinkles, problems of degradation of various kinds of film properties on the coating film occur. The thicker the film thickness of a cured film, the stronger such a tendency is.
In addition, when a cured film, in other words, a coating film of an ink which is cured, from an ultraviolet curable ink jet ink using a recording method which is disclosed in International Publication No. WO 2011/039081, is attempted to be formed, problems occur as below.
Firstly, in a case where the film thickness of a cured film is relatively thin, the hardenability is inferior due to the affects of oxygen inhibition in a case of a radical polymerization reaction system. Therefore, the film has to be thickened in excess to the extent in which oxygen inhibition does not occur when printing and a problem of the printed image quality occurs where the image becomes very poor. On the other hand, in a case where the film thickness of a cured film is relatively thick, there is a strong tendency that a pigment absorbs a part of ultraviolet light, and then, even if ultraviolet light is irradiated, the energy which is needed for completely curing the coating film which is discharged onto a recording medium, becomes insufficient, therefore, there is a case where the curing of the inside of the coating film becomes incomplete due to the vicinity of the surface of the coating film being cured in first or there is a need to take time for curing. Further, due to the vicinity of the surface which has been cured before wrinkles when an uncured ink which is present inside the coating film is cured or an ink irregularly flowing before an ink inside the coating film is cured, wrinkles (hereinafter also referred to as “cured wrinkles”) occur on the surface of the film coating after curing. Due to the cured wrinkles, problems occur wherein film characteristics on the coating film are inferior.